MySQL Query fix and speed up

I'm newish to MySQL and a lot of my query is pieced together from this site and trial and error, so any help is appreciated. If I left out anything, or more detail is needed, I will be happy to give what I can.

My query takes, on average, about 90 secs to run. It also doesn't return all the correct values. Even if speed can't be fixed, how do I correct it to return the proper values?

I'm trying to get the most recent set of temperatures for a given location and also an average for the last 24hrs. A location can have up to 20 thermometers.

SELECT t.Common_name, f.Food_name, e.Temperature, e.Temp_timestamp AS Time, Daily_avg.avg_temp
FROM Thermometer t
JOIN Temp_records e 
  ON e.Temp_therm_id = t.Therm_id
LEFT JOIN Food f 
  ON f.Food_therm_id = t.Therm_id
JOIN (
  SELECT Temp_therm_id, AVG( Temperature ) AS avg_temp
  FROM Temp_records
  WHERE Temp_timestamp >= NOW( ) - INTERVAL 1 DAY
  GROUP BY Temp_therm_id
) AS Daily_avg
USING ( Temp_therm_id )
WHERE t.Therm_unit_id =2
AND Temp_records_id
IN (
  SELECT MAX( Temp_records_id )
  FROM Temp_records
  GROUP BY Temp_therm_id
)

Thermometer Table

Therm_id 
Therm_unit_id 
Common_name 

Temp_records Table (~18k records at the moment)

Temp_records_id
Temp_therm_id
Temp_food_id
Temperature 

Food Table

Food_id
Food_unit_id
Food_name
Food_therm_id   

What I'm currently seeing:

(The probe isn't in food, that's just the ambient temperature in C)
The db has slightly different temperatures for the given food items, and since I can only get the temperature of one food item at a time, they obviously can't have the same time stamp.

Should I remove the averaging part in to it's own query?

If I remove the food part of the query, it returns my 3 most recent temperatures and their averages. (I only have 3 thermometers for testing at the moment)